This invention relates in general to systems and methods for generating optical beams and in particular to systems and methods for generating nearly non-diffracting optical beams.
Any field of wavelength X initially confined to a finite area of radius r in a transverse plane will be subject to diffractive spreading as it propagates outward from that plane into free space. The diffractive spreading becomes increasingly noticeable at distances beyond the Rayleigh range of scale r2/λ. Diffraction sets a limit that makes it impossible to simultaneously obtain a beam with very narrow waist (r is very small) and a long non-diffracting propagation distance (longer than the Rayleigh range). Although this limit holds for most beam shapes including Gaussian beams, Durnin showed the possibility of diffraction-free beams with special field distributions, so-called Bessel beams, see, J. Durnin, “Exact solutions for nondiffracting beams. I. The scalar theory,” J. Opt. Soc. Am. A, vol. 4, pp. 651-654, April 1987, whose central spot can be extremely narrow, on the order of one wavelength, without being subject to diffractive spreading. Bessel beams have attracted substantial interest because of their various applications such as optical acceleration, particle guiding and manipulation, nonlinear optics, optical interconnection and alignment, imaging, microfabrication, and lithography.
Axicons see, J. H. Mcleod, “The axicon: a new type of optical element,” J. Opt. Soc. Am., vol. 44, pp. 592-597, August 1954, G. Indebetouw, “Nondiffracting optical fields: some remarks in their analysis and synthesis,” J. Opt. Soc. Am. A, vol. 6, pp. 150-152, 1989, are the best-known and most common tool for generating Bessel beams. But they are bulk optics, free-space elements and require careful alignment. Although microaxicons fabricated on the fiber end via chemically etching see, S. K. Eah and W. Jhe, “Nearly diffraction-limited focusing of a fiber axicon microlens,” Rev. Sci. Instrum., vol. 74, pp. 4969-4971, 2003, focused ion beam machining see S. Cabrini, C. Liberale, D. Cojoc, A. Carpentiero, M. Prasciolou, S. Mora, V. Degiorgio, F. De Angelis, and E. Di Fabrizio, “Axicon lens on optical fiber forming optical tweezers, made by focused ion beam milling,” Microelectron. Eng. Vol. 83, pp. 804-807, 2006, and mechanically polishing see, T. Grosjean, S. S. Saleh, M. A. Suarez, I. A. Ibrahim, V. Piquerey, D. Charraut., and P. Sandoz, “Fiber microaxicons fabricated by a polishing technique for the generation of Bessel-like beams,” Appl. Opt, vol. 46, pp. 8061-8067, November 2007, can overcome the disadvantages of bulk axicons and offer the possibility of producing Bessel-like beams with a compact fiber device, their manufacturing process is costly, complicated, and difficult to control. Very recently, Ramachandran see, S. Ramachandran and S. Ghalmi, “Diffraction-free, self-healing Bessel beams from fibers,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science conference, San Jose, 2008, CPDB5, demonstrated a novel method to generate Bessel-like beam from an optical fiber with a long-period fiber Bragg grating inscribed into its core. The beam shape, however, is strongly wavelength-dependent and special equipment is needed to fabricate the grating.
It s therefore desirable to provide improved systems and methods for generating nearly non-diffracting optical beams with better characteristics. The standard by which the characteristics of nearly non-diffracting optical beams is measured is by comparison to a Gaussian beam.